Subsea hydrocarbon wells in deep water initially have enough natural or reservoir pressure to flow the well fluids to a wellhead at the sea floor, plus up a riser or flow line to a processing facility at the sea surface. The reservoir pressure declines over time, and eventually becomes inadequate to lift the well fluid to the surface processing facility, which may be thousands of feet above the sea floor. Even though the well may have sufficient pressure to lift the column to the sea floor, it may have to be closed in unless some type of artificial lift is employed.
Well submersible pumps are commonly used in land-based wells to pump the well fluid to the wellhead when the reservoir pressure is inadequate. One type of submersible well pump is an electrical submersible pump (ESP), which normally employs a three-phase electrical motor to drive a centrifugal pump. In most installations, the ESP is supported on a string of production tubing extending into the well. ESPs are capable of not only lifting the column of well fluid to the wellhead, but if installed in a subsea well, also up a riser or flow line to a production facility. However, ESPs have to be pulled from the well from time to time for maintenance or replacement. In deep water, pulling an ESP from a subsea well is very expensive. Normally, a semi-submersible drilling rig is required to pull the production tubing and the ESP from a well. Consequently, operators are reluctant to install ESPs in deep water subsea wells.
Sea floor pumps have been proposed to boost the pressure of the well fluid flowing out of the wellhead. A sea floor pump lifts the column of well fluid from the sea floor to a production facility at the surface. However, sea floor pumps are also quite expensive if installed in deep water.
Both land-based and subsea wells have used a technique known as gas lift to enhance production of a well. In one technique, a gas lift mandrel will be secured in the production tubing. The gas lift mandrel has a port leading from the tubing annulus surrounding the production tubing to the interior of the production tubing. A check valve can be lowered on a wireline through the tubing and installed in the gas lift mandrel. The operator pumps compressed gas into the tubing annulus, which flows through the check valve into the column of well fluid in the production tubing. The injected gas lightens the column of well fluid in the tubing, facilitating flow to the well head. A drawback to subsea gas lift is the requirement for a gas source and compressor to inject the gas into the tubing annulus. In deep water, the gas source and compressor would likely need to be located on the sea floor. The cost may be too much for deep water offshore wells.